Comet Alert

HEADS up! Here comes a comet. That's the word from amateur observers who expect a striking display this spring. We've heard this cry before only to have the promised show flop when the comet failed to perform. But this time the advance signs are quite favorable.

Rodney R.D. Austin of New Plymouth, New Zealand, discovered the comet that now bears his name Dec. 6, 1989. Within a month, the astronomy magazine Sky & Telescope alerted news media that this celestial visitor could be a showoff. For Mr. Austin to see it with an amateur telescope while it was still 230 million miles from the sun means the comet is probably very large. It could easily become visible to the naked eye, although that can't be guaranteed, the magazine said in its advisory.

Now, after weeks of observing, the approaching comet continues to show its promise of a spectacular display.

Comet Austin will swing around the sun April 9 at a distance of about 32 million miles. Amateur observer John E. Bortle notes in Sky & Telescope's March issue that only three similar comets have come that close to the sun since 1950. All three put on good displays. And Comet Austin already is four to five times brighter in its absolute magnitude than the average comet.

Residents of the Northern Hemisphere will have the best view. Even for them, however, the comet will be elusive as it approaches the sun. It will lie low in the western sky at dusk. Careful observers with binoculars may be able to pick it up from mid-March onward. But from mid-April through May - after it swings around the sun - the comet will be a pre-dawn object well above the horizon. It should grow a majestic tail that sets it off from the stars and be easily visible as the mid-April full moon wanes.

The best viewing is likely to be in late April and the first week of May. The comet will have faded a bit when it comes closest to Earth, passing by at about 23 million miles on May 25.

Solar system scientists have learned a great deal about comets during the past decade, especially as spacecraft from several nations made close observations of Comets Giacobini-Zinner and Halley in 1985 and 1986. They have confirmed the ``dirty snowball'' model that envisions a comet as being a mass of ice and dust with perhaps a rocky core. Comets also have a variety of minor constituents, including organic molecules.

In fact, organic material from space raining down on the primitive Earth may have helped organic life get started on our planet. Edward Anders, a University of Chicago chemist and meteorite analyst, has shown that certain compounds, including some amino acids, could have arrived in significant quantities from space. Much of this resource would have come in on dust particles thrown off by comets.

Comet researchers now look forward to what the United States National Aeronautics and Space Administration calls one of its highest-priority scientific missions. This is the Comet Rendezvous and Asteroid Flyby scheduled for launch in 1995. The spacecraft is to intercept Kopff - a comet several miles in diameter - somewhere between Jupiter and Mars in the year 2000 and fly with it toward the sun. The spacecraft will orbit the comet for a year, study it with a battery of sensors, and look for a landing site. Then it will drop a lander with a penetrating probe onto the comet's surface. It will continue to fly with Kopff until 2003.

Comet Austin won't get that kind of scientific attention. But it should be worth a trip to a clear viewing area early on a springtime morning.